State of the Art
Automatic in-line electric water heaters are known in which the water to be heated is brought into heat absorbing relation to an electrical resistance element which is automatically subjected to the heating current by the flow of the water to be heated. In U.S. Pat. No. 1,057,760 to Miles, a suitably housed piston is moved to close an electrical switch against a spring bias by the pressure of water from a valved supply pipe. The water then passes through a restricted pipe to the mid-height of a vertical cylindrical casing containing bare wire heating coils positioned in a vertical insulating tube open at its ends. The water passes downwardly about the lower portion of the tube and then upwardly in contact with the resistance wires in the tube and then to a delivery spout. The water pressure actuated switch supplies current to the water-immersed electric heating wires. This device leaves much to be desired from the standpoint of safety and reliability and is subject to clogging by sediment collecting in the bottom of the cylindrical casing.
More recently a similar device has been available in which the vertical cylinder has been provided with a removable drain plug at its lower end and a supporting plug at its top, and the bare wire resistance coils and insulating tube have been replaced by an assembly carried by the supporting plug of a U-shaped heating element of the so-called "cal-rod" type having its heating elements embedded in insulating material in a rigid copper tube, and an asbestos partition member positioned to extend within the vertical cylinder between the legs of the U-shaped heating element to cause the water to flow down one side of the partition, under the same, and upwardly on the other side thereof, to a restricted outlet opening, to which a shower bath spray nozzel is attached, a pressure-actuated switch being supported on the supporting plug in a removable metal housing at the top of the device, and communicating with the interior of the cylinder through the plug and controlling the supply of current to the heating element from a standard three conductor cable connected to the switch and heater elements within the removable metal housing. This more recent device represents an improvement over the Miles device but still has disadvantages due to clogging, need to remove the drain plug for clean out, and exposure of electrical connections if one removes the metal housing which could produce a hazardous situation.